Just want to add that we had the same problem with Odie and clipping nails. It was the only time she would ever snap at us, and you could tell it was coming from a place of fear and self preservation. She was terrified. She never would actually bite us, but she was definitely threatening and it got to the point where we thought that we would have to wear gloves.

We tried most of what everyone has suggested here and nothing worked. She was too stressed to even be interested in a treat, and it wouldn't matter if one person was holding her, as she'd go for any hand that was closest. I never tried getting someone else to clip them, because I was honestly scared of someone actually getting bit. We would have to put a cone on her head to get the job done and she would still be thrashing around, which made for a very long process.

Anyway, we can now cut her nails without any aggression or snapping! It still takes one person holding her and the other cutting her nails, but that's mainly just to get better access. In the midst of our troubles, I came across an article on Cesar Millan's website and we decided to give their advice a go and it totally worked for us. The link is here. Odie did a short swim in our bathtub (with me holding her since she's not the best swimmer), and when we took her out we cut her nails without incident and they were nice and soft and it went quickly. We were able to do them all after the one short swim, and gave her a treat and it was over! Ever since then, we've been able to do it no prob. Not sure if this would work for your dog, since he has aggression issues other than nail cutting, but thought I would share!